The invention relates to an assembly for signal transfer between a receiving station and a transmitting station as well as for power supply of the transmitting station.
Known from EP-A-0 744 724 is one such circuit assembly in which the two stations are connected by a two-wire line via which an analog signal current variable between two limiting values is transmitted, this signal current representing a measured value sensed by a sensor in the transmitting station and forming the supply current needed to operate the transmitting station. In this arrangement the transmitting station comprises a circuit generating a constant operating voltage for the transmitting station, and it includes a controllable current source which dictates the current flowing via the two-wire line as a function of the measured value and which is fed from a supply voltage source in the receiving station, wherein the transmitting station xe2x80x9cseesxe2x80x9d at its two inputs a voltage which may vary over wide ranges depending on the selection of the supply voltage source. For transferring the measured value the transmitting station regulates its input current ideally so that it depends solely on the measured value. In this arrangement the supply of the transmitting station is made exclusively via the two-wire line, the input voltage generally being higher than the supply voltage required internally. This is why in the transmitting station the input voltage is stepped down by a linear regulator to the supply voltage required internally. This unfortunately limits the available supply current by the input current of the transmitting station, it being this limitation that restricts the flexibility as regards use of sensors and signal analyzing circuits in the transmitting station, since there is a desire too, to use sensors requiring a current higher than that supplied to it via the two-wire line.
It is thus the object of the invention to provide a signal transfer and power supply assembly which as regards the sensors and signal conditioning units used in the transmitting station is highly flexible and as regards the power supply can be tailored to individual circumstances.
To achieve this object the invention provides an assembly for signal transfer between a receiving station and a transmitting station as well as for power supply of the transmitting station in which these two stations are connected to each other by a two-wire line, via which an analog signal current variable between two limiting values is transmitted, this signal current representing a measured value sensed by a sensor in the transmitting station and forming the supply current needed to operate the transmitting station. In this arrangement the transmitting station comprises a circuit generating a constant operating voltage for the transmitting station, and it includes a controllable current source which dictates the current flowing via the two-wire line as a function of the measured value and which is fed from a supply voltage source in the receiving station, the current source being a series current regulator fed from the supply voltage source in the receiving station, a charge pump being connected to the output of the current source, the charge pump generating from the voltage appearing at the output of the current source the operating voltage needed for operating the sensor and a signal processing circuit connected thereto, and a parallel regulator for maintaining the input voltage or respectively the output voltage of the charge pump constant being connected to the input and output respectively of the charge pump.
In accordance with a first aspect of the invention, on the corresponding other side of the charge pump a further parallel regulator is provided in addition to the parallel regulator at the input or output of the charge pump.
In accordance with a second aspect of the invention, on the corresponding other side of the charge pump a linear regulator is provided in addition to the parallel regulator at the input or output of the charge pump.
In one improvement the current source is bridged by a voltage regulator furnishing in a starting phase an input voltage for the charge pump, the current source being configured so that it does not furnish an output current in the starting phase until the charge pump outputs an output voltage sufficient for its operation, the voltage regulator being configured so that it changes to the OFF state as soon as the output voltage has attained the operating voltage.
In another improvement the charge pump has a voltage transfer factor  less than 1.
By using a combination of current and voltage regulators in conjunction with a charge pump the current and voltage values needed for operating the transmitting station may be set over wide ranges so that a high flexibility is attained as regards the usable sensors. More particularly, circuit units may be put to use in the transmitting station requiring a supply current higher than the current permitted to flow as a maximum signal current via the two-wire line to the receiving station. Likewise a salient advantage of the assembly in accordance with the invention is its facilitated integratability, it containing no inductances, but instead capacitors in the main which are easy to produce integrated with capacitances  less than 1 nF.